"insert tube" method for blown-in, what tube?

I have a Dutch colonial with a gambrel and figure I can blow insulation into the second story walls from the attic using the "insert tube" method but I don't know what I should use as the tube. It needs to be flexible to bend into the wall cavity yet stiff enough to push to the bottom of the cavity. Can anyone suggest what to use for this tube?
Thanks, Scott

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